U.S. patent application number 13/551230 was filed with the patent office on 2012-11-08 for nanowire-based membrane electrode assemblies for fuel cells.
This patent application is currently assigned to NANOSYS, INC.. Invention is credited to Calvin Y.H. Chow, Stephen A. Empedocles, Chunming Niu, J. Wallace Parce.
Application Number | 20120282540 13/551230 |
Document ID | / |
Family ID | 36578478 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120282540 |
Kind Code |
A1 |
Niu; Chunming ; et
al. |
November 8, 2012 |
Nanowire-Based Membrane Electrode Assemblies for Fuel Cells
Abstract
The present invention discloses nanowires for use in a fuel cell
comprising a metal catalyst deposited on a surface of the
nanowires. A membrane electrode assembly for a fuel cell is
disclosed which generally comprises a proton exchange membrane, an
anode electrode, and a cathode electrode, wherein at least one or
more of the anode electrode and cathode electrode comprise an
interconnected network of the catalyst supported nanowires. Methods
are also disclosed for preparing a membrane electrode assembly and
fuel cell based upon an interconnected network of nanowires.
Inventors: |
Niu; Chunming; (Palo Alto,
CA) ; Chow; Calvin Y.H.; (Portola Valley, CA)
; Empedocles; Stephen A.; (Los Altos, CA) ; Parce;
J. Wallace; (Palo Alto, CA) |
Assignee: |
NANOSYS, INC.
Palo Alto
CA
|
Family ID: |
36578478 |
Appl. No.: |
13/551230 |
Filed: |
July 17, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13149527 |
May 31, 2011 |
|
|
|
13551230 |
|
|
|
|
12234104 |
Sep 19, 2008 |
7977007 |
|
|
13149527 |
|
|
|
|
11642241 |
Dec 20, 2006 |
7977013 |
|
|
12234104 |
|
|
|
|
11295133 |
Dec 6, 2005 |
7179561 |
|
|
11642241 |
|
|
|
|
60738100 |
Nov 21, 2005 |
|
|
|
60634472 |
Dec 9, 2004 |
|
|
|
Current U.S.
Class: |
429/483 ;
429/484 |
Current CPC
Class: |
B82Y 30/00 20130101;
Y10S 977/762 20130101; Y02P 70/56 20151101; H01M 4/881 20130101;
H01M 4/921 20130101; H01M 8/1011 20130101; H01M 4/8853 20130101;
H01M 4/90 20130101; H01M 8/0236 20130101; H01M 8/0234 20130101;
H01M 8/023 20130101; H01M 8/1007 20160201; H01M 4/9075 20130101;
Y10S 977/948 20130101; Y02E 60/50 20130101; H01M 4/926 20130101;
H01M 4/8846 20130101; H01M 4/8605 20130101; H01M 8/0241 20130101;
H01M 8/1004 20130101; Y02P 70/50 20151101; Y02E 60/523 20130101;
H01M 8/1086 20130101; H01M 4/925 20130101; H01M 4/8867 20130101;
H01M 8/0232 20130101 |
Class at
Publication: |
429/483 ;
429/484 |
International
Class: |
H01M 4/96 20060101
H01M004/96; H01M 8/10 20060101 H01M008/10 |
Claims
1. A composition comprising: a plurality of nanowires attached
directly to a support structure comprising graphite, carbon or a
carbon composition, the nanowires comprising crystalline silicon,
polycrystalline silicon, amorphous silicon, or a mixture thereof;
and an electrolyte.
2. The composition of claim 1, wherein the nanowires are dispersed
within the electrolyte.
3. The composition of claim 1, wherein a portion of the electrolyte
is attached to the nanowires.
4. The composition of claim 1, wherein a portion of the electrolyte
is directly linked to the surface of the nanowires.
5. The composition of claim 1, wherein a portion of the electrolyte
forms a continuous or discontinuous film coating on the surface of
the nanowires.
6. The composition of claim 1, wherein a portion of the electrolyte
is uniformly wetted on the surface of the nanowires.
7. The composition of claim 1, wherein the nanowires comprise an
interconnected network of nanowires.
8. The composition of claim 7, wherein the nanowires comprise at
least one surface chemical group which forms chemical cross-links
between the interconnected nanowires.
9. The composition of claim 1, wherein the electrolyte comprises a
polymer electrolyte comprising an ionomer, a polyethylene oxide, a
poly(ethylene succinate), a poly(beta-propiolactone), or a
sulfonated fluoropolymer.
10. The composition of claim 1, wherein the surfaces of the
nanowires are functionalized.
11. The composition of claim 10, wherein the nanowires are
functionalized with at least one functional group which promotes
wetting of the electrolyte on the nanowires.
12. The composition of claim 10, wherein the nanowires are
functionalized with at least one functional group which binds the
electrolyte to the nanowires.
13. The composition of claim 10, wherein the nanowires are
functionalized with at least one molecule selected from the group
consisting of: a short chain polymer, a sulfonated hydrocarbon, a
short chain sulfonated hydrocarbon, a fluorocarbon, a short chain
fluorocarbon, and a branched hydrocarbon.
14. The composition of claim 10, wherein the nanowires are
functionalized with at least one functional group selected from the
group consisting of: a nitric acid group, a carboxylic acid group,
a hydroxyl group, an amine group, a sulfonic acid group, or a
silane group.
15. The composition of claim 1, wherein the composition is porous,
wherein the porous composition comprises pores disposed between the
nanowires of the plurality of nanowires.
16. The composition of claim 15, wherein the electrolyte is
disposed within the pores of the porous composition.
17. (canceled)
18. The composition of claim 1, wherein the nanowires comprise a
core and one or more shell layers disposed on the core.
19. The composition of claim 1, further comprising one or more
metal catalyst particles attached to the nanowires, wherein the
metal catalyst particles comprise gold, copper, or mixtures or
alloys thereof.
20. The composition of claim 1, wherein the nanowires are grown on
the support structure.
21. The composition of claim 1, wherein the composition is
electrically conductive.
22. An electrode comprising the composition of claim 1.
23. An anode comprising the composition of claim 1.
24. An article, comprising: an anode; a cathode; an electrolyte,
and wherein the anode comprises a substrate and a plurality of
nanowires attached directly to the substrate, the substrate
comprises graphite, carbon or a carbon composition, and the
nanowires comprise crystalline silicon, polycrystalline silicon,
amorphous silicon, or a mixture thereof.
25. The article of claim 24, wherein the nanowires are grown on the
substrate.
26. The article of claim 24, wherein a portion of the electrolyte
is directly linked to the surface of the nanowires.
27. The article of claim 24, wherein a portion of the electrolyte
forms a continuous or discontinuous film coating on the surface of
the nanowires.
28. The article of claim 24, wherein the nanowires are
functionalized with at least one functional group which binds the
electrolyte to the nanowires.
29. The article of claim 24, wherein the substrate is porous.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This non-provisional application is a continuation of U.S.
patent application Ser. No. 13/149,527, filed May 31, 2011, which
is a continuation of U.S. patent application Ser. No. 12/234,104,
filed Sep. 19, 2008, now U.S. Pat. No. 7,977,007, which is a
continuation of U.S. patent application Ser. No. 11/642,241, filed
Dec. 20, 2006, now U.S. Pat. No. 7,977,013, which is a continuation
of U.S. patent application Ser. No. 11/295,133, filed Dec. 6, 2005,
now U.S. Pat. No. 7,179,561, which claims priority to U.S.
provisional Patent Application No. 60/738,100, filed Nov. 21, 2005,
and U.S. provisional Patent Application No. 60/634,472, filed Dec.
9, 2004, the entire contents of each of which are incorporated by
reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] This invention relates to fuel cells generally, and, more
particularly, to nanowire-based electrodes and membrane electrode
assemblies for such fuel cells.
BACKGROUND OF THE INVENTION
[0004] Fuel cells are devices that convert the chemical energy of
fuels, such as hydrogen and methanol, directly into electrical
energy. The basic physical structure or building block of a fuel
cell consists of an electrolyte layer in contact with a porous
anode and cathode on either side. A schematic representation of a
fuel cell with the reactant/product gases and the ion conduction
flow directions through the cell is shown in FIG. 1. In a typical
fuel cell as shown in FIG. 1, a fuel (e.g., methanol or hydrogen)
is fed to an anode catalyst that converts the fuel molecules into
protons (and carbon dioxide for methanol fuel cells), which pass
through the proton exchange membrane to the cathode side of the
cell. At the cathode catalyst, the protons (e.g., hydrogen atoms
without an electron) react with the oxygen ions to form water. By
connecting a conductive wire from the anode to the cathode side,
the electrons stripped from fuel, hydrogen or methanol on the anode
side can travel to the cathode side and combine with oxygen to form
oxygen ions, thus producing electricity. Fuel cells operating by
electrochemical oxidation of hydrogen or methanol fuels at the
anode and reduction of oxygen at the cathode are attractive power
sources because of their high conversion efficiencies, low
pollution, lightweight, and high energy density.
[0005] For example, in direct methanol fuel cells (DMFCs), the
liquid methanol (CH.sub.3OH) is oxidized in the presence of water
at the anode generating CO.sub.2, hydrogen ions and the electrons
that travel through the external circuit as the electric output of
the fuel cell. The hydrogen ions travel through the electrolyte and
react with oxygen from the air and the electrons from the external
circuit to form water at the anode completing the circuit.
[0006] Anode Reaction: CH.sub.3OH+H.sub.2O=>CO.sub.2+6H++6e-
[0007] Cathode Reaction: 3/2 O.sub.2+6 H++6e-=>3 H.sub.2O
[0008] Overall Cell Reaction: CH.sub.3OH+3/2 O.sub.2=>CO.sub.2+2
H.sub.2O
[0009] Initially developed in the early 1990s, DMFCs were not
embraced because of their low efficiency and power density, as well
as other problems. Improvements in catalysts and other recent
developments have increased power density 20-fold and the
efficiency may eventually reach 40%. These cells have been tested
in a temperature range from about 50.degree. C.-120.degree. C. This
low operating temperature and no requirement for a fuel reformer
make the DMFC an excellent candidate for very small to mid-sized
applications, such as cellular phones, laptops, cameras and other
consumer products, up to automobile power plants. One of the
drawbacks of the DMFC is that the low-temperature oxidation of
methanol to hydrogen ions and carbon dioxide requires a more active
catalyst, which typically means a larger quantity of expensive
platinum (and/or ruthenium) catalyst is required.
[0010] A DMFC typically requires the use of ruthenium (Ru) as a
catalyst component because of its high carbon monoxide (CO)
tolerance and reactivity. Ru disassociates water to create an
oxygenated species that facilitates the oxygenation of CO, which is
produced from the methanol, to CO.sub.2. Some existing DFMCs use
nanometer-sized bimetallic Pt:Ru particles as the electro-oxidation
catalyst because of the high surface area to volume ratio of the
particles. The Pt/Ru nanoparticles are typically provided on a
carbon support (e.g., carbon black, fullerene soot, or desulfurized
carbon black) to yield a packed particle composite catalyst
structure. Most commonly used techniques for creating the Pt:Ru
carbon packed particle composite are the impregnation of a carbon
support in a solution containing platinum and ruthenium chlorides
followed by thermal reduction
[0011] A multi-phase interface or contact is established among the
fuel cell reactants, electrolyte, active Pt:Ru nanoparticles, and
carbon support in the region of the porous electrode. The nature of
this interface plays a critical role in the electrochemical
performance of the fuel cell. It is known that only a portion of
catalyst particle sites in packed particle composites are utilized
because other sites are either not accessible to the reactants, or
not connected to the carbon support network (electron path) and/or
electrolyte (proton path). In fact, current packed particle
composites only utilize about 20 to 30% of the catalyst particles.
Thus, most DMFCs which utilize packed particle composite structures
are highly inefficient.
[0012] In addition, connectivity to the anode and/or cathode is
currently limited in current packed particle composite structures
due to poor contacts between particles and/or tortuous diffusion
paths for fuel cell reactants between densely packed particles.
Increasing the density of the electrolyte or support matrix
increases connectivity, but also decreases methanol diffusion to
the catalytic site. Thus, a delicate balance must be maintained
among the electrode, electrolyte, and gaseous phases in the porous
electrode structure in order to maximize the efficiency of fuel
cell operation at a reasonable cost. Much of the recent effort in
the development of fuel cell technology has been devoted to
reducing the thickness of cell components while refining and
improving the electrode structure and the electrolyte phase, with
the aim of obtaining a higher and more stable electrochemical
performance while lowering cost. In order to develop commercially
viable DFMCs, the electrocatalytic activity of the catalyst must be
improved.
[0013] The present invention meets these and other needs as well.
The present invention generally provides a novel nanowire composite
membrane electrode catalyst support assembly that provides a highly
porous material with a high surface area, a high structural
stability and a continuum structure. The composite structure may be
provided as a highly interconnected nanowire supported catalyst
structure interpenetrated with en electrolyte network to maximize
catalyst utilization, catalyst accessibility, and electrical and
ionic connectivity to thereby improve the overall efficiency of
fuel cells, at lower cost, etc.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention provides a proton exchange membrane
fuel cell with nanostructured components, in particular, one or
more of the electrodes of the membrane electrode assembly. The
nanostructured fuel cell has a higher catalytic metal utilization
rate at the electrodes, higher power density (kW/volume and
kW/mass), and lower cost than conventional fuel cells. The
nanostructured fuel cells are not only attractive for stationary
and mobile applications, but also for use as a compact power supply
for microelectronics such as laptops, cell phones, cameras and
other electronic devices.
[0015] In accordance with a first aspect of the present invention,
nanowires (e.g., inorganic nanowires) for use in a membrane
electrode assembly of a fuel cell are disclosed which generally
comprise a metal catalyst deposited on a surface of the nanowires.
The metal catalyst may be deposited as a thin film on the surface
of the nanowires, or as a layer of catalyst particles, e.g., by
functionalizing the surface of the nanowires with standard surface
chemistries. The metal catalyst may be selected from the group
comprising one or more of platinum (Pt), ruthenium (Ru), iron (Fe),
cobalt (Co), gold (Au), chromium (Cr), molybdenum (Mo), tungsten
(W), manganese (Mn), technetium (Tc), rhenium (Re), osmium (Os),
rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), copper
(Cu), silver (Ag), zinc (Zn), tin (Sn), aluminum (Al), and
combinations and alloys thereof (such as bimetallic Pt:Ru
nanoparticles). The nanowires may comprise branched structures
(e.g., side nodules) to increase the surface area to volume ratio
of the wires to maximize the catalytic efficiency of the fuel cell.
The nanowires may be made from metallic conducting, semiconducting,
carbide, nitride, or oxide materials such as RuO.sub.2, SiC, GaN,
TiO.sub.2, SnO.sub.2, WC.sub.x, MoC.sub.x, ZrC, WN.sub.x, MoN.sub.x
etc. It is preferable that the nanowires be made from a material
that is resistant to degradation in a weak acid so that the
nanowires are compatible with the reactants of a variety of
different fuel cells.
[0016] The nanowires may be derivatized with at least a first
functional group or chemical binding moiety which binds to metallic
catalyst particles, such as a nitric acid group, carboxylic acid
group, a hydroxyl group, an amine group, a sulfonic acid group, and
the like, or the catalyst may be deposited as a thin film using
other deposition processes such as electrodeposition, atomic layer
deposition, plasma sputtering, etc. The nanowires may also be
derivatized with a functional group which differentially binds to a
thin proton conducting polymer coating (e.g., Nafion.RTM. or other
sulfonated polymer) which may be deposited directly on the
nanowires. For example, the nanowires may be functionalized with a
sulfonated hydrocarbon, fluorocarbon, or branched hydrocarbon chain
using known standard chemistries. Alternatively, instead of binding
ionomer to the nanowires through a chemical binding moiety, the
nanowires may be functionalized to make them proton conductive. For
example, the nanowires may be functionalized with a surface coating
such as a perfluorinated sulfonated hydrocarbon using well-known
functionalization chemistries. In this way, the intimate
relationship between the nanowire catalyst support and the polymer
shell ensures that most, if not all, of the metal catalyst
particles are located at a three-phase contact point (e.g., such
that the catalyst particles are accessible to the fuel cell
reactants, electrolyte and nanowire core for efficient electron and
proton conduction). The controlled nanowire surface chemistry can
be used to control the wettability of the polymer in the composite
nanowire structure and ensures that catalyst particles are exposed
and accessible for catalysis.
[0017] According to another embodiment of the present invention, a
nanostructured catalyst support for a membrane electrode assembly
of a fuel cell is disclosed which generally comprises an
interconnected mat or network of nanowires each having a metal
catalyst deposited thereon. The catalyst metal may comprise any of
the catalyst metals previously disclosed such as platinum. The
catalyst metal may comprise a combination of metals such as
platinum and ruthenium. In one representative embodiment, the
catalyst metal comprises nanoparticles having a diameter less than
about 50 nm, e.g., less than about 10 nm, e.g., less than about 5
nm, e.g., between about 1 and 5 nm. In this embodiment, each
nanowire in the network of nanowires typically is physically and/or
electrically connected to at least one or more other nanowires in
the nanowire network to form a highly interconnected network of
nanowires. In other embodiments, the nanowires may be substantially
aligned in a parallel array of nanowires between the anode/cathode
bipolar plates and the proton exchange membrane, or the nanowires
may be randomly oriented. The nanowires may each be coated with a
first catalyst colloid coating and/or a second thin proton
conducting polymer coating (e.g., Nafion.RTM.). The membrane
electrode assembly may be a component in a direct methanol fuel
cell, a hydrogen fuel cell, or any other fuel cell known to those
of ordinary skill in the art.
[0018] A fuel cell is formed by providing a proton exchange
membrane, an anode electrode, a cathode electrode, and first and
second bipolar plates, wherein at least one of the anode and
cathode electrode comprise an interconnected network of the
catalyst supported nanowires. Because of the superior connectivity
of the nanowire network, the fuel cell may not require a gas
diffusion layer between the proton exchange membrane and the first
or second bipolar plates as is the case with conventional fuel
cells. In one embodiment, the nanowires may be synthesized directly
on one or more of the bipolar plates of the fuel cell and/or on the
proton exchange membrane. The nanowires may also be grown on a
separate growth substrate, harvested therefrom, and then
transferred (e.g., as a porous sheet of interconnected wires) and
incorporated into the fuel cell structure (e.g., deposited on one
or more of the fuel cell components such as one or more of the
bipolar plates and/or the proton exchange membrane). When grown in
situ on the bipolar plate(s) and/or proton exchange membrane, the
nanowires may be oriented substantially perpendicular or normal to
a surface of the bipolar plate(s) or proton exchange membrane, or
oriented randomly.
[0019] The nanowires in the nanowire network are preferentially
physically and/or electrically connected to one or more other wires
in the network to form an open, highly branched, porous,
intertwined structure, with low overall diffusion resistance for
reactants and waste diffusion, high structural stability and high
electrical connectivity for the electrons to ensure high catalytic
efficiency, thus leading to high power density and lower overall
cost. The multiple electrical connectivity of the nanowires ensures
that if one wire breaks or is damaged in the system, for example,
that all points along the wire still connect to the anode (or
cathode) electrode along different paths (e.g., via other nanowires
in the network). This provides substantially improved electrical
connectivity and stability as compared to previous packed particle
composite structures. The catalyst is highly accessible to the fuel
source to produce electrons and protons, while the electrons can
conduct directly to the bipolar plate through the nanowire and the
protons can transport directly to the membrane through the
polymer.
[0020] The nanowires in the network of nanowires may be
cross-linked or fused together using various cross-linking or
sintering methods described further herein at points where such
nanowires contact or are proximal to others of the nanowires to
increase the connectivity and structural stability of the nanowire
network. In another embodiment, the same strategy of cross-linking
or sintering can be used to improve the electrical or structural
connectivity between the nanowires and catalyst material that is in
contact or proximal with such nanowires.
[0021] The nanowire network defines a plurality of pores between
the nanowires in the network, wherein the plurality of pores
preferentially have an effective pore size of less than about 10
.mu.m, for example, less than about 5 .mu.m, e.g., less than about
1 .mu.m, e.g., less than about 0.2 .mu.m, e.g., less than 0.02
.mu.m, e.g., between about 0.002 .mu.m and 0.02 .mu.m, e.g.,
between about 0.005 and 0.01 .mu.m. The overall porosity of the
branched nanowire structure may be greater than about 30%, for
example, between about 30% and 95%, e.g., between about 40% and
60%. The nanowires are dispersed in a porous polymer matrix
electrolyte material such as perfluorosulfonic acid/PTFE copolymer
(e.g., Nafion.RTM.) which forms a continuous network
interpenetrated with the nanowires in the branched nanowire network
to provide sufficient contact points for proton (e.g., H+)
transport.
[0022] In another embodiment of the present invention, a method for
preparing a fuel cell membrane electrode is disclosed which
generally comprises (a) associating a catalyst metal selected from
the group comprising one or more of chromium (Cr), molybdenum (Mo),
tungsten (W), manganese (Mn), technetium (Tc), rhenium (Re), iron
(Fe), ruthenium (Ru), osmium (Os), cobalt (Co), rhodium (Rh),
iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper
(Cu), silver (Ag), gold (Au), zinc (Zn), tin (Sn), aluminum (Al),
and combinations thereof, with a plurality of inorganic nanowires
to form a plurality of inorganic nanowires with associated catalyst
metal, and (b) forming a membrane electrode comprising a plurality
of inorganic nanowires with associated catalyst metal.
[0023] The plurality of inorganic nanowires may be derivatized with
at least a first functional group which binds the catalyst metal
such as a nitric acid group, a carboxylic acid group, a hydroxyl
group, an amine group, a sulfonic acid group, and the like. The
associating may also be done by a variety of methods selected from
the group comprising chemical vapor deposition, electrochemical
deposition, physical vapor deposition, solution impregnation and
precipitation, colloid particle absorption and deposition, atomic
layer deposition, and combinations thereof. For example, the
associating may be done by chemical deposition of a catalyst metal
precursor such as chloroplatinic acid or by electrodeposition of Pt
from a precursor salt in solution. The catalyst metal precursor may
be converted to a catalytically active metal by subjecting the
catalyst metal precursor to metal reduction, wherein metal
reduction is done by a method selected from the group comprising
hydrogen reduction, chemical reduction, electrochemical reduction
and a combination thereof. The catalytically active metal may be in
the form of metal nanoparticles on the surface of the nanowires.
The forming may be done on a proton exchange membrane or on one or
more of the bipolar plates, for example, by a method selected from
the group comprising spray/brush painting, solution coating,
casting, electrolytic deposition, filtering a fluid suspension of
the nanowires, and combinations thereof. The nanowires may also be
grown directly on one or more of the fuel cell components such as
one or more of the bipolar plates and/or proton exchange membrane.
The method may further comprise mixing an ionomeric resin (e.g.,
perfluorosulfonic acid/PTFE copolymer, e.g., Nafion) with the
plurality of inorganic nanowires with associated catalyst metal.
The plurality of inorganic nanowires may be derivatized with at
least a second functional group (e.g., a sulfonated hydrocarbon
group) which binds the ionomeric resin.
[0024] In another embodiment of the present invention, a method of
making a membrane electrode assembly of a fuel cell is disclosed
which generally comprises: forming nanowires on a growth substrate;
transferring the nanowires from the growth substrate into a fluid
suspension; depositing one or more catalyst metals on the nanowires
to form a nanowire supported catalyst; filtering the fluid
suspension of nanowires to create a porous sheet of interconnected
nanowires; infiltrating the network of nanowires with an ionomeric
resin; and combining the sheet of interconnected nanowires with a
proton exchange membrane to form a membrane electrode assembly
(MEA). Hot pressing may be used to fuse electrolyte in both the
anode and cathode electrode with the proton exchange membrane to
form a continuous electrolyte phase for efficient proton transport
from the anode electrode to the cathode electrode. The step of
depositing one or more catalyst metals may comprise, for example,
depositing a metal selected from the group comprising platinum,
gold, ruthenium, and other metals, and combinations thereof. The
method may further comprise forming a proton exchange membrane fuel
cell utilizing the formed MEA by combining first and second bipolar
plates together to form the proton exchange membrane fuel cell.
[0025] For a further understanding of the nature and advantages of
the invention, reference should be made to the following
description taken in conjunction with the accompanying figures. It
is to be expressly understood, however, that each of the figures is
provided for the purpose of illustration and description only and
is not intended as a definition of the limits of the embodiments of
the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic representation of a conventional
electrochemical fuel cell showing exemplary reactions in the anode
and the cathode electrodes;
[0027] FIG. 2A is an expanded view of the anode electrode portion
of the fuel cell of FIG. 1 showing details of a conventional packed
particle composite catalyst structure comprising Pt/Ru
nanoparticles provided on a carbon particle support;
[0028] FIG. 2B is an expanded view of the packed particle composite
catalyst structure of FIG. 2A showing an exemplary three-phase
contact between the gaseous reactants, electrolyte, and the
electrocatalyst structure;
[0029] FIG. 3A is a schematic representation of a nanowire-based
electrochemical fuel cell made according to the teachings of the
present invention;
[0030] FIG. 3B is a schematic representation of a nanowire-based
electrochemical fuel cell stack made according to the teachings of
the present invention
[0031] FIG. 4A is an expanded view of the anode electrode portion
of the fuel cell of FIG. 3 showing details of an embodiment of an
interconnected network of catalyst supported nanowires which span
the junction between the proton exchange membrane and anode
electrode of the fuel cell of FIG. 3;
[0032] FIG. 4B is an expanded view of an alternative embodiment for
a nanowire-based anode portion of a fuel cell showing details of a
parallel array of catalyst supported nanowires which span the
junction between the proton exchange membrane and the anode
electrode of the fuel cell of FIG. 3;
[0033] FIG. 5 is a SEM image of an interconnected network of
nanowires used as the catalyst support in an anode (and/or cathode)
electrode of a fuel cell made according to the teachings of the
present invention.
[0034] FIG. 6 is a schematic representation of a branched nanowire
structure that can be used in practicing the methods of the present
invention;
[0035] FIG. 7 is an SEM image of a branched nanowire network
including a plurality of branched nanowires having tiny nodules
extending from the side surfaces of the nanowires;
[0036] FIG. 8 is an SEM image at high magnification of cross-linked
or fused nanowires creating an interconnecting nanowire network as
used in certain aspects of the present invention.
[0037] FIG. 9 is a SEM image showing Au catalyst particles
deposited on a network of interconnected nanowires.
DETAILED DESCRIPTION OF THE INVENTION
[0038] The membrane electrode assemblies and fuel cells of the
present invention gain significant unique properties by
incorporating nanowires in their component structures. The term
"nanowire" generally denotes an elongated structure having an
aspect ratio (length:width) of greater than 10, preferably greater
than 100 and in many cases 1000 or higher. These nanowires
typically have a cross sectional dimension, e.g., a diameter that
is less than 500 nm and preferably less than 100 nm and in many
cases, less than 50 nm, e.g., above 1 nm.
[0039] The composition of the nanowires employed in the invention
may vary. By way of example, nanowires may be comprised of organic
polymers, ceramics, inorganic semiconductors such as carbides and
nitrides, and oxides (such as TiO.sub.2 or ZnO), carbon nanotubes,
biologically derived compounds, e.g., fibrillar proteins, etc. or
the like. For example, in certain embodiments, inorganic nanowires
are employed, such as semiconductor nanowires. Semiconductor
nanowires can be comprised of a number of Group IV, Group III-V or
Group II-VI semiconductors or their oxides. In one embodiment, the
nanowires may include metallic conducting, semiconducting, carbide,
nitride, or oxide materials such as RuO.sub.2, SiC, GaN, TiO.sub.2,
SnO.sub.2, WC.sub.x, MoC.sub.x, ZrC, WN.sub.x, MoN.sub.x etc. It is
preferable that the nanowires be made from a material that is
resistant to degradation in a weak acid so that the nanowires are
compatible with are compatible with the reactants of a variety of
different fuel cells. Nanowires according to this invention can
expressly exclude carbon nanotubes, and, in certain embodiments,
exclude "whiskers" or "nanowhiskers", particularly whiskers having
a diameter greater than 100 nm, or greater than about 200 nm.
[0040] Typically, the nanowires employed are produced by growing or
synthesizing these elongated structures on substrate surfaces. By
way of example, published U.S. Patent Application No.
US-2003-0089899-A1 discloses methods of growing uniform populations
of semiconductor nanowires from gold colloids adhered to a solid
substrate using vapor phase epitaxy. Greene et al.
("Low-temperature wafer scale production of ZnO nanowire arrays",
L. Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R.
Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031-3034, 2003)
discloses an alternate method of synthesizing nanowires using a
solution based, lower temperature wire growth process. A variety of
other methods are used to synthesize other elongated nanomaterials,
including the surfactant based synthetic methods disclosed in U.S.
Pat. Nos. 5,505,928, 6,225,198 and 6,306,736, for producing shorter
nanomaterials, and the known methods for producing carbon
nanotubes, see, e.g., U.S. Pat. No. 7,416,699 to Dai et al., as
well as methods for growth of nanowires without the use of a growth
substrate, see, e.g., Morales and Lieber, Science, V. 279, p. 208
(Jan. 9, 1998). As noted herein, any or all of these different
materials may be employed in producing the nanowires for use in the
invention. For some applications, a wide variety of group III-V,
II-VI and group IV semiconductors may be utilized, depending upon
the ultimate application of the substrate or article produced. In
general, such semiconductor nanowires have been described in, e.g.,
U.S. Pat. No. 7,301,199, incorporated herein above. In certain
embodiments, the nanowires are selected from a group consisting of:
Si, Ge, Sn, Se, Te, B, Diamond, P, B-C, B-P(BP6), B--Si, Si--C,
Si--Ge, Si--Sn and Ge--Sn, SiC, BN/BP/BAs, AlN/AlP/AlAs/AlSb,
GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, BN/BP/BAs, AlN/AlP/AlAs/AlSb,
GaN/GaP/GaAs/GaSb, InN/InP/InAs/InSb, ZnO/ZnS/ZnSe/ZnTe,
CdS/CdSe/CdTe, HgS/HgSe/HgTe, BeS/BeSe/BeTe/MgS/MgSe, GeS, GeSe,
GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI,
AgF, AgCl, AgBr, AgI, BeSiN.sub.2, CaCN.sub.2, ZnGeP.sub.2,
CdSnAs.sub.2, ZnSnSb.sub.2, CuGeP.sub.3, CuSi.sub.2P.sub.3, (Cu,
Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te).sub.2, Si.sub.3N.sub.4,
Ge.sub.3N.sub.4, Al.sub.2O.sub.3, (Al, Ga, In).sub.2(S, Se,
Te).sub.3, Al.sub.2CO, and an appropriate combination of two ore
more such semiconductors.
[0041] In the cases of semiconductor nanowires, the nanowires may
optionally comprise a dopant to increase the conductivity of the
nanowire catalyst support. The dopant may be selected from a group
consisting of: a p-type dopant from Group III of the periodic
table; an n-type dopant from Group V of the periodic table; a
p-type dopant selected from a group consisting of: B, Al and In; an
n-type dopant selected from a group consisting of: P, As and Sb; a
p-type dopant from Group II of the periodic table; a p-type dopant
selected from a group consisting of: Mg, Zn, Cd and Hg; a p-type
dopant from Group IV of the periodic table; a p-type dopant
selected from a group consisting of: C and Si; or an n-type is
selected from a group consisting of: Si, Ge, Sn, S, Se and Te.
[0042] Additionally, such nanowires may be homogeneous in their
composition, including single crystal structures, or they may be
comprised of heterostructures of different materials, e.g.,
longitudinal heterostructures that change composition over their
length, or coaxial heterostructures that change composition over
their cross section or diameter. Such coaxial and longitudinal
heterostructured nanowires are described in detail in, e.g.,
Published International Patent Application No. WO 02/080280, which
is incorporated herein by reference for all purposes.
[0043] Furthermore, as disclosed in greater detail in co-pending,
co-assigned provisional Patent Application 60/738,100, filed Nov.
21, 2005, the entire contents of which are incorporated by
reference herein, nanowire structures with multiple shells can also
be fabricated, such as, for example, a conducting inner core wire
(which may or may not be doped) (e.g., to impart the necessary
conductivity for electron transport) and one or more outer-shell
layers that provide a suitable surface for binding catalyst (and/or
polymer electrolyte). For example, in one embodiment, a multi-layer
or multi-walled carbon nanotube (MWNT) can be formed in which the
outermost shell layer is converted to silicon carbide to provide a
surface (SiC) to bind catalyst (and/or polymer electrolyte) and a
conductive carbon nanotube core to impart the necessary
conductivity. In alternative embodiments, the core may consist of
heavily doped material such as doped silicon, and a shell of a
carbide, nitride etc. material (e.g., SiC) may then be formed on
the core. The use of silicon as the core material leverages the
extensive experience and infrastructure known for fabricating
silicon nanowires. A carbide shell, such as SiC, WC, MoC or mixed
carbide (e.g. WSiC) may be formed around the core material using a
controlled surface reaction. SiC, WC and MoC are known for their
high conductivity and chemical stability. In addition, these
materials have been shown to have catalytic properties similar to
those of precious metals, such as Pt, for methanol oxidation, and
therefore may provide further performance enhancements in the
nanowire bird's nest MEA. The precursor materials for the shell may
be deposited on the core nanowire surface (e.g., silicon) by atomic
layer deposition (ALD) and then converted to the carbide by
high-temperature carbothermal reduction, for example.
[0044] Synthesis of core-shell nanowire (and other nanocrystal)
heterostructures are described in, e.g., Berkeley U.S. Pat. No.
6,996,147; co-assigned U.S. Pat. No. 7,339,184; Peng et al. (1997)
"Epitaxial growth of highly luminescent CdSe/CdS core/shell
nanocrystals with photostability and electronic accessibility" J.
Am. Chem. Soc. 119, 7019-7029; Dabbousi et al. (1997) "(CdSe)ZnS
core-shell quantum dots: Synthesis and characterization of a size
series of highly luminescent nanocrysallites" J. Phys. Chem. B 101,
9463-9475; Manna et al. (2002) "Epitaxial growth and photochemical
annealing of graded CdS/ZnS shells on colloidal CdSe nanorods" J.
Am. Chem. Soc. 124, 7136-7145, the entire contents of each of which
are incorporated by reference herein. Similar approaches can be
applied to the growth of other core-shell nanostructures including
nanowires.
[0045] In one embodiment of the invention, the nanowire portion of
the anode (and/or cathode) electrode of the invention may be
synthesized on a growth substrate, and then transferred and
incorporated into the membrane electrode assembly structure of the
fuel cell. For example, in certain aspects, inorganic semiconductor
or semiconductor oxide nanowires are grown on the surface of a
growth substrate using a colloidal catalyst based VLS synthesis
method described above. In accordance with this synthesis
technique, the colloidal catalyst (e.g., gold, platinum etc.
particles) is deposited upon the desired surface of the substrate.
The substrate including the colloidal catalyst is then subjected to
the synthesis process which generates nanowires attached to the
surface of the substrate. Other synthetic methods include the use
of thin catalyst films, e.g., 50 nm or less, deposited over the
surface of the substrate. The heat of the VLS process then melts
the film to form small droplets of catalyst that forms the
nanowires. Typically, this latter method may be employed where
fiber diameter homogeneity is less critical to the ultimate
application. Typically, catalysts comprise metals, e.g., gold or
platinum, and may be electroplated or evaporated onto the surface
of the substrate or deposited in any of a number of other well
known metal deposition techniques, e.g., sputtering etc. In the
case of colloid deposition the colloids are typically deposited by
first treating the surface of the substrate so that the colloids
adhere to the surface. Such treatments include those that have been
described in detail previously, i.e., polylysine treatment, etc.
The substrate with the treated surface is then immersed in a
suspension of colloid.
[0046] Following growth of the nanowires, the nanowires are then
harvested from their synthesis location. The free standing
nanowires are then introduced into or deposited upon the relevant
surface of the fuel cell component such as the bipolar plate(s) or
proton exchange membrane, for example, by a method selected from
spray/brush painting, solution coating, casting, electrolytic
deposition, filtering a fluid suspension of the nanowires, and
combinations thereof. For example, such deposition may simply
involve immersing the component of interest (e.g., one or more of
the bipolar plates or the proton exchange membrane) into a
suspension of such nanowires, or may additionally involve
pre-treating all or portions of the component to functionalize the
surface or surface portions for wire attachment. As described
further below, the nanowires may also be introduced into a solution
(e.g., methanol or water), filtered (e.g., vacuum filtered over a
polyvinylidene fluoride (PVDF) membrane) to give them a dense,
intertwined mat or "bird's nest structure," removed from the filter
after drying and washing, and then heat treated (e.g., annealed) at
high temperatures. The resulting porous sheet of interconnected
nanowires can then be incorporated into the membrane electrode
assembly of the fuel cell. A variety of other deposition methods,
e.g., as described in U.S. Pat. No. 7,067,328, and U.S. Pat. No.
6,962,823, the full disclosures of which are incorporated herein by
reference in their entirety for all purposes. As explained further
below, the nanowires may also be grown directly on one or more of
the fuel cell components such as one or more of the bipolar plates
and/or proton exchange membrane.
[0047] Typically, as shown in FIG. 1, a fuel cell 100 generally
comprises an anode electrode 102, a cathode electrode 104, and a
proton exchange membrane (PEM) 106. The assembly of these three
components is generally referred to as a membrane electrode
assembly (MEA). As described previously, if methanol is used as
fuel, liquid methanol (CH.sub.3OH) is oxidized in the presence of
water at the anode 102 generating CO.sub.2, hydrogen ions and the
electrons that travel through the external circuit 108 as the
electric output of the fuel cell. The hydrogen ions travel through
the electrolyte membrane 106 and react with oxygen from the air and
the electrons from the external circuit 108 to form water at the
cathode completing the circuit. Anode and cathode electrodes 102,
104 each contact bipolar plates 110, 112, respectively. The bipolar
plates 110, 112 typically have channels and/or grooves in their
surfaces that distribute fuel and oxidant to their respective
catalyst electrodes, allow the waste, e.g., water and CO.sub.2 to
get out, and may also contain conduits for heat transfer.
Typically, bipolar plates are highly electrically conductive and
can be made from graphite, metals, conductive polymers, and alloys
and composites thereof. Materials such as stainless steel, aluminum
alloys, carbon and composites, with or without coatings, are good
viable options for bipolar end plates in PEM fuel cells. Bipolar
plates can also be formed from composite materials comprising
highly-conductive or semiconducting nanowires incorporated in the
composite structure (e.g., metal, conductive polymer etc.). The
shape and size of the components of the fuel cell can vary over a
wide range depending on the particular design.
[0048] In another embodiment, nanowires may be deposited (e.g.,
grown) on one or more of the bipolar plates to provide a high
surface area electrode plate with low flow resistance for methanol
(or other fuel cell gas or liquid reactants) and waste products
through it. A more complete description of nanowire structures
having enhanced surface areas, as well as to the use of such
nanowires and nanowire structures in various high surface area
applications, is provided in U.S. Ser. No. 10/792,402 entitled
"Nanofiber Surfaces for use in Enhanced Surface Area Applications,"
filed Mar. 2, 2004, the entire contents of which are incorporated
by reference herein.
[0049] At present, the most commonly used electrode catalyst is Pt
or Pt:Ru particles 202 supported on carbon particles 204 (e.g.,
made from carbon black) which are dispersed in an electrolyte film
206 as shown in the expanded view of the anode 102 in FIG. 2A. One
of the challenges in the commercialization of proton exchange
membrane fuel cells (PEMFCs) is the high cost of the precious
metals used as the catalyst (e.g., Pt or Ru). Decreasing the amount
of Pt used in a PEMFC by increasing the utilization efficiency of
Pt has been one of the major concerns during the past decade. To
effectively utilize the Pt catalyst, the Pt should have
simultaneous contact to the reactant gases (or reactant solutions
or liquids), the electrolyte (e.g., proton conducting film), and
the carbon particles (e.g., electron-conducting element). As shown
in FIG. 2B, an effective electrode in a fuel cell requires a
4-phase-contact 208 in the catalyst layer between the reactant
gases/liquid, active metal particles, carbon support 202, 204, and
the electrolyte 206. A preferred catalyst layer allows the facile
transport of reactant gases (e.g., methanol, MeOH:H.sub.2O,
hydrogen and/or oxygen), solutions, or liquids, facile transport of
electrons to/from the external circuit and protons to/from the
proton exchange membrane.
[0050] The carbon particles conduct electrons and the
perfluorosulfonate ionomer (e.g., Nafion.RTM.) conducts protons. As
noted previously, in conventional packed particle composite systems
as shown in FIGS. 2A-B, there is a significant portion of Pt (or
Pt:Ru) that is isolated from the external circuit and/or the PEM,
resulting in a low Pt utilization. For example, current packed
particle composites only utilize about 20 to 30% of the catalyst
particles. The inaccessibility to some catalyst sites can be due,
for example, to the fact that the necessary addition of the
solubilized perfluorosulfonate ionomer (e.g., Nafion.RTM.) for
proton transport tends to wash away or isolate carbon particles in
the catalyst layer, leading to poor electron transport. Thus, most
DMFCs which utilize packed particle composite structures are highly
inefficient.
[0051] Due to their unique structural, mechanical, and electrical
properties, the inventors of the present application have
discovered that nanowires can be used to replace traditional carbon
particles in PEMFCs as the catalyst support and electron conducting
medium to make MEAs. Because the generation of surface functional
groups on nanowires, e.g., nanowires such as SiC or GaN, is
relatively straightforward, catalyst nanoparticles such as Pt
and/or Pt:Ru (as well as a proton conducting polymer (e.g.,
Nafion)), can be facilely deposited on the nanowires, e.g., without
agglomeration of the particles. Each catalyst particle is then
directly connected to the anode (and cathode) through the nanowire
core. The multiple electrical connectivity of the interconnected
nanowires secures the electronic route from Pt to the electron
conducting layer. The use of nanowires and the resulting guaranteed
electronic pathway eliminate the previously mentioned problem with
conventional PEMFC strategies where the proton conducting medium
(e.g., Nafion) would isolate the carbon particles in the electrode
layer. Eliminating the isolation of the carbon particles supporting
the electrode layer improves the utilization rate of Pt.
[0052] As shown now with reference to FIG. 3A, a nanowire-based
fuel cell is shown which includes an anode bipolar electrode plate
302, a cathode bipolar electrode plate 304, a proton exchange
membrane 306, an anode electrode 308, a cathode electrode 310, and
an interconnecting network of nanowires 312 positioned between both
the anode electrode 308 and cathode electrode 310 on one side, and
the proton exchange membrane 306 on the other side of the fuel
cell. Generally, a plurality of fuel cells or MEAs as shown in FIG.
3A can be combined to form a fuel cell stack as shown, for example,
in FIG. 3B having separate anode electrodes 308, 320 and cathode
electrodes 310, 322 separated by respective proton exchange
membranes 306 and 306', respectively. The cells within the stacks
are connected in series by virtue of the bipolar plates 302, 304,
318, and 324 such that the voltages of the individual fuel cells
are additive.
[0053] As shown in FIGS. 3A, 4A and in the SEM image of FIG. 5, the
nanowires 316 in the nanowire networks 312 each are physically
and/or electrically connected to one or more other wires in the
network to form an open, highly branched, porous, intertwined
structure, with low overall diffusion resistance for reactants and
waste diffusion, high structural stability and high electrical
connectivity for the electrons to ensure high catalytic efficiency,
thus leading to high power density and lower overall cost. It is
important to note that even if two wires are not in actual direct
physical contact with each other (or with a catalyst particle), it
is possible that at some small distance apart, they may still be
able to transfer changes (e.g., be in electrical contact).
Preferentially, each nanowire is physically and/or electrically
connected to at least one or more other nanowire in the network.
The multiple connectivity of the nanowires ensures that if one wire
breaks or is damaged in the system, for example, that all points
along the wire still connect to the anode (and cathode) electrode
along different paths (e.g., via other nanowires in the network).
This provides substantially improved electrical connectivity and
stability as compared to previous packed particle composite
structures. The wires may extend all the way (or only part way)
between the anode (and cathode) bipolar plate and the proton
exchange membrane. In the case where the wires do not extend all
the way between a bipolar plate and the membrane, the wires may
extend from the bipolar plate toward the membrane, but not reach
the membrane, and the polymer electrolyte can extend from the
membrane toward the bipolar plate, but not reach the bipolar plate
(but not the other way around) to ensure that electrons are
efficiently transferred to the anode, and protons are transferred
towards the cathode.
[0054] The nanowires in the nanowire network may optionally have a
branched structure and include a plurality of nodules 600 which
extend from side surfaces of the nanowire as shown in FIG. 6 and in
the SEM image of FIG. 7. The nodules 600 on the sides of the
nanowire core can further increase available surface area for
catalysis without substantially impacting the connectivity or
porosity of the nanowire network.
[0055] The nanowires 316 are dispersed in a polymer electrolyte
material 315 (e.g., see FIG. 4A) which coats the surface of
nanowires in the branched nanowire network to provide sufficient
contact points for proton (e.g., H+) transport. Polymer
electrolytes can be made from a variety of polymers including, for
example, polyethylene oxide, poly (ethylene succinate), poly
(beta.-propiolactone), and sulfonated fluoropolymers such as
Nafion.RTM. (commercially available from DuPont Chemicals,
Wilmington). A suitable cation exchange membrane is described in
U.S. Pat. No. 5,399,184, for example, incorporated herein by
reference. Alternatively, the proton conductive membrane can be an
expanded membrane with a porous microstructure where an ion
exchange material impregnates the membrane effectively filling the
interior volume of the membrane. U.S. Pat. No. 5,635,041,
incorporated herein by reference, describes such a membrane formed
from expanded polytetrafluoroethylene (PTFE). The expanded PTFE
membrane has a microstructure of nodes interconnected by fibrils.
Similar structures are described in U.S. Pat. No. 4,849,311,
incorporated herein by reference.
[0056] The porous structure of the interconnected nanowire network
provides an open (non-tortuous) diffusion path for fuel cell
reactants to the catalyst (e.g., catalyst particles 314) deposited
on the nanowires 316 as described further below. The void spaces
between the interconnected nanowires form a highly porous
structure. The effective pore size will generally depend upon the
density of the nanowire population, as well as the thickness of
electrolyte layer, and to some extent, the width of the nanowires
used. All of these parameters are readily varied to yield a
nanowire network having a desired effective porosity. For example,
preferred nanowire networks have a porosity adequate to provide for
an even flow of reactants while maintaining adequate electrical
conductivity and mechanical strength. Also, the porosity of the
nanowire network provides for water management within the cell. The
branched nanowire network preferably is sufficiently porous to pass
fuel gases and water vapor through it without providing a site for
water condensation that would block the pores of the network and
prevent vapor transport. The mean pore size generally ranges from
about 0.002 microns to about 10.0 microns, e.g., less than about 1
.mu.m, e.g., less than about 0.2 .mu.m, e.g., less than about 0.02
.mu.m, e.g., between about 0.002 .mu.m and 0.02 .mu.m, e.g.,
between about 0.005 and 0.01 .mu.m. The total porosity of the
branched nanowire structure may be easily controlled between about
30% to 95%, for example, e.g., between about 40% to 60%, while
still ensuring electrical connectivity to the anode and cathode
electrodes.
[0057] The nanowires 316 which form the interconnected nanowire
networks 312 may optionally be fused or cross-linked at the points
where the various wires contact each other, to create a more
stable, robust and potentially rigid membrane electrode assembly.
The nanowires may also include surface chemical groups that may
form chemical cross-links in order to cross-link the underlying
nanowires. For example, the nanowires may be cross-linked or fused
together by depositing a small amount of conducting or
semiconducting material at their cross-points. For example, SiC
nanowires (or, e.g., carbon nanotube nanowires having a SiC shell
layer) can be cross-linked by depositing amorphous or
polycrystalline SiC at their cross-points. FIG. 8 is an SEM
micrograph showing a plurality of silicon nanowires which have been
fused together using deposited polysilicon at their cross-points.
One of skill in the art will appreciate that other metals,
semimetals, semiconductors, and semiconductor oxides could also be
used to cross-link these intersections.
[0058] In another aspect of the present invention shown with
reference to FIG. 4B, nanowires 316' may be provided as a parallel
array of aligned wires having electrolyte 315' interspersed between
the free spaces between the aligned wires. In this particular
implementation of the present invention, the parallel array of
nanowires is preferably synthesized in situ, e.g., on the surface
of the bipolar electrode plate(s) 302 and/or 304 (and/or the proton
exchange membrane 306). It is to be understood that the randomly
oriented, interconnected network 312 of wires 316 shown in FIGS.
3A, 4A and 5 and described above can also be grown in situ directly
on the bipolar plates 302, 304 (and/or proton exchange membrane)
using the techniques described herein. For example, inorganic
semiconductor or semiconductor oxide nanowires may be grown
directly on the surface of the electrode plate using a colloidal
catalyst based VLS synthesis method described above. In accordance
with this synthesis technique, the colloidal catalyst is deposited
upon the desired surface of the bipolar plate. The bipolar plate
including the colloidal catalyst is then subjected to the synthesis
process which generates nanowires attached to the surface of the
plate. Other synthetic methods include the use of thin catalyst
films, e.g., 50 nm or less, deposited over the surface of the
bipolar plate. The heat of the VLS process then melts the film to
form small droplets of catalyst that forms the nanowires.
Typically, this latter method may be employed where wire diameter
homogeneity is less critical to the ultimate application.
Typically, catalysts comprise metals, e.g., gold of platinum, and
may be electroplated or evaporated onto the surface of the
electrode plate or deposited in any of a number of other well known
metal deposition techniques, e.g., sputtering etc. In the case of
colloid deposition the colloids are typically deposited by first
treating the surface of the electrode plate so that the colloids
adhere to the surface. The plate with the treated surface is then
immersed in a suspension of colloid.
[0059] In another aspect of the invention, the anode electrode 308
(and cathode electrode 310) may include a conductive grid or mesh
made from any of a variety of solid or semisolid materials such as
organic materials, e.g., conductive polymers, carbon sheets, etc.,
inorganic materials, e.g., semiconductors, metals such as gold,
semimetals, as well as composites of any or all of these, upon
which the nanowires 316 may be attached, but through which
apertures exist. Such meshes provide relatively consistent surfaces
in a ready available commercial format with well defined
screen/pore and wire sizes. A wide variety of metal meshes are
readily commercially available in a variety of such screen/pore and
wire sizes. Alternatively, metal substrates may be provided as
perforated plates, e.g., solid metal sheets through which apertures
have been fabricated. Fabricating apertures in meal plates may be
accomplished by any of a number of means. For example relatively
small apertures, e.g., less than 100 .mu.m in diameter, may be
fabricated using lithographic and preferably photolithographic
techniques. Similarly, such apertures may be fabricated using laser
based techniques, e.g., ablation, laser drilling, etc. For larger
apertures, e.g., greater than 50-100 .mu.m, more conventional metal
fabrication techniques may be employed, e.g., stamping, drilling or
the like. As formed, the metal grids or meshes with the nanowires
formed or deposited thereon by the methods disclosed herein may be
deposited on the proton exchange membrane, bipolar plate(s), and or
embedded within one or more of the electrode layers to provide a
porous network with a high surface area nanowire catalyst support
attached thereto for efficient catalysis. Other examples of a
variety grids or meshes with nanowires deposited thereon which can
be used in the present invention are fully disclosed in U.S. patent
application Ser. No. 10/941,746, entitled "Porous Substrates,
Articles, Systems and Compositions Comprising Nanofibers and
Methods of Their Use and Production," filed on Sep. 15, 2004, the
entire contents of which are incorporated by reference herein.
[0060] The nanowire network thus formed by any of the previously
disclosed methods is employed as the support for the subsequent
metal (e.g., platinum, ruthenium, gold, or other metal described
below) catalyst, which may be coated or deposited, for example, on
the nanowires. Appropriate catalysts for fuel cells generally
depend on the reactants selected. For example, the metallic
catalyst may be selected from the group comprising one or more of
platinum (Pt), ruthenium (Ru), iron (Fe), cobalt (Co), gold (Au),
chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn),
technetium (Tc), rhenium (Re), osmium (Os), rhodium (Rh), iridium
(Ir), nickel (Ni), palladium (Pd), copper (Cu), silver (Ag), zinc
(Zn), tin (Sn), aluminum (Al), and combinations and alloys thereof
(such as bimetallic Pt:Ru nanoparticles). Suitable catalyst
materials for oxidation of hydrogen or methanol fuels specifically
include metals such as, for example, Pd, Pt, Ru, Rh and alloys
thereof.
[0061] The catalyst may be deposited or otherwise associated with
the nanowire surface as a thin film (e.g., less than about 10
angstroms in thickness) (or a series of catalyst particles) by
using a variety of catalyst deposition techniques including, for
example, chemical vapor deposition, electrochemical deposition
(e.g., electroplating or electroless chemical plating), physical
vapor deposition, solution impregnation and precipitation, colloid
particle absorption and deposition, atomic layer deposition, and
combinations thereof. The amount of the catalyst metal coated by
the methods described above is preferably in the range of about
10-85% by weight, more preferably, 20-40% by weight, based on the
total amount of catalyst metal and nanowire material.
[0062] Alternatively, in one particular embodiment as shown with
reference to FIGS. 3A and 4A-B, the catalyst may be deposited on
the nanowire surface in solution as a plurality of nanometer-sized
metallic catalyst particles 314 (e.g., between about 1 and 50 nm in
diameter, e.g., less than about 10 nm in diameter, e.g., between
about 1 and 5 nm in diameter), e.g., by derivatizing the nanowire
external surface with one or more functional linker moieties (e.g.,
a chemically reactive group) such as one or more carboxylic acid
groups, nitric acid groups, hydroxyl groups, amine groups, sulfonic
acid groups, and the like. The catalysts particles (or film) can be
attached to the wires either uniformly or non-uniformly. The
catalyst particles can be spherical, semi-spherical or
non-spherical. The catalyst particles can form islands on the
surface of the nanowires or can form a continuous coating on the
surface of the nanowire such as in a core-shell arrangement, or
stripes or rings along the length of the nanowire, etc. The
catalyst particles may be attached to the nanowire surface before
or after the nanowire network is incorporated/deposited into the
MEA of the fuel cell. In one embodiment, the catalyst particles may
be selected from a population of catalyst particles having a
uniform size distribution of less than about 50%, for example, less
than about 30%, for example, less than about 20%.
[0063] When a chemical linker molecule is used to bind the catalyst
to the nanowire, the chemical linker can be selected to promote
electrical connection between the catalyst and the wire, or the
chemical linker can be subsequently removed to promote electrical
connection. For example. heat, vacuum, chemical agents or a
combination thereof, may optionally be applied to the nanowires to
cause the linker molecule to be removed to place the catalyst in
direct physical contact with the wire to form a solid electrical
connection between the catalyst particles and the nanowire. The
structure can also be heated to anneal the interface between the
catalyst and the wire in order to improve the electrical contact
therebetween.
[0064] In addition to the conductive catalyst particles, fillers
can be used to alter the physical properties of the nanowire
composite structures useful in the invention. Appropriate fillers
include, e.g. silica (SiO.sub.2), powdered polytetrafluoroethylene
and graphite fluoride (CF.sub.n). The polymer films preferably can
include up to about 20 percent by weight fillers, and more
preferably from about 2 to about 10 percent by weight fillers. The
fillers are generally in the form of particles.
[0065] Following catalyst deposition, a proton conducting polymer
such as Nafion may optionally be deposited on the nanowire surface
between catalyst particle sites, for example, by functionalizing
the surface of the nanowire with a second functional group
(different from the catalyst functional group, when used) that
preferentially binds the electrolyte or which promotes consistent
and/or controlled wetting. The polymer can either be a continuous
or discontinuous film on the surface of the nanowire. For example,
the polymer electrolyte can be uniformly wetted on the surface of
the wires, or can form point-contacts along the length of the wire.
The nanowires may be functionalized with a sulfonated hydrocarbon
molecule, a fluorocarbon molecule, a short chain polymer of both
types of molecules, or a branched hydrocarbon chain which may be
attached to the nanowire surface via silane chemistry. Those of
skill in the art will be familiar with numerous functionalizations
and functionalization techniques which are optionally used herein
(e.g., similar to those used in construction of separation columns,
bio-assays, etc.). Alternatively, instead of binding ionomer to the
nanowires through a chemical binding moiety, the nanowires may be
directly functionalized to make them proton conductive. For
example, the nanowires may be functionalized with a surface coating
such as a perfluorinated sulfonated hydrocarbon using well-known
functionalization chemistries.
[0066] For example, details regarding relevant moiety and other
chemistries, as well as methods for construction/use of such, can
be found, e.g., in Hermanson Bioconjugate Techniques Academic Press
(1996), Kirk-Othmer Concise Encyclopedia of Chemical Technology
(1999) Fourth Edition by Grayson et al. (ed.) John Wiley &
Sons, Inc., New York and in Kirk-Othmer Encyclopedia of Chemical
Technology Fourth Edition (1998 and 2000) by Grayson et al. (ed.)
Wiley Interscience (print edition)/John Wiley & Sons, Inc.
(e-format). Further relevant information can be found in CRC
Handbook of Chemistry and Physics (2003) 83.sup.rd edition by CRC
Press. Details on conductive and other coatings, which can also be
incorporated onto the nanowire surface by plasma methods and the
like can be found in H. S. Nalwa (ed.), Handbook of Organic
Conductive Molecules and Polymers, John Wiley & Sons 1997. See
also, "ORGANIC SPECIES THAT FACILITATE CHARGE TRANSFER TO/FROM
NANOCRYSTALS," U.S. Pat. No. 6,949,206. Details regarding organic
chemistry, relevant for, e.g., coupling of additional moieties to a
functionalized surface can be found, e.g., in Greene (1981)
Protective Groups in Organic Synthesis, John Wiley and Sons, New
York, as well as in Schmidt (1996) Organic Chemistry Mosby, St
Louis, Mo., and March's Advanced Organic Chemistry Reactions,
Mechanisms and Structure, Fifth Edition (2000) Smith and March,
Wiley Interscience New York ISBN 0-471-58589-0, and U.S. Pat. No.
7,985,475. Those of skill in the art will be familiar with many
other related references and techniques amenable for
functionalization of surfaces herein.
[0067] The polymer electrolyte coating may be directly linked to
the surface of the nanowires, e.g., through silane groups, or may
be coupled via linker binding groups or other appropriate chemical
reactive groups to participate in linkage chemistries
(derivitization) with linking agents such as, e.g., substituted
silanes, diacetylenes, acrylates, acrylamides, vinyl, styryls,
silicon oxide, boron oxide, phosphorus oxide,
N-(3-aminopropyl)-3-mercapto-benzamide,
3-aminopropyl-trimethoxysilane, 3-mercaptopropyl-trimethoxysilane,
3-maleimidopropyl-trimethoxysilane,
3-hydrazidopropyl-trimethoxysilane, trichloro-perfluoro octyl
silane, hydroxysuccinimides, maleimides, haloacetyls, hydrazines,
ethyldiethylamino propylcarbodiimide, and/or the like. Other
surface functional chemistries can be used such as those that would
be known to one or ordinary skill in the art.
[0068] In addition, a solubilized perfluorosulfonate ionomer (e.g.,
Nafion) may be placed into the spare space between nanowires. The
composite nanowire structure (e.g., as a porous sheet of
interconnected nanowires, e.g., made by the process described in
the Example below), when not grown in situ on one of the bipolar
plates and/or proton exchange membrane, may then be placed between
bipolar plates on either side of a proton exchange membrane, and
the assembly hot pressed to form a complete membrane-electrode
assembly fuel cell according to the present invention. The pressing
temperature is determined such that the proton exchange membrane is
softened in that temperature range, for example, to 125 degrees
Celsius for Nafion. The pressure level is about 200 kgf/cm.sup.2.
In order to efficiently distribute fuel/oxygen to the surface of
the anode/cathode electrodes 308, 310, a gas diffusion layer is
typically needed in conventional fuel cells between the anode
electrode and bipolar plate on one side, and the cathode electrode
and bipolar plate on the other side of the fuel cell. Typically, a
carbon fiber cloth is used as the gas diffusion layer. With the
interconnecting nanowire composite membrane electrode catalyst
support assembly of the present invention, this gas diffusion layer
can be eliminated due to the superior structure of the
nanowire-based electrodes.
Example
[0069] The following non-limiting example describes an exemplary
process for depositing gold (Au) nanoparticles on the surface of
nanowires for use in a membrane electrode assembly according to the
teachings of the present invention.
[0070] Approximately 10 mg Si nanowires were dispersed in ethanol
by sonication to form a nanowire suspension. An interconnected
nanowire network was prepared by vacuum filtration of the nanowire
suspension over a polyvinylidene fluoride (PVDF) membrane and
vacuum drying, then 2 cc 0.1% polylysine solution was added to the
filter funnel to absorb polylysine on the surface of the nanowires.
After 5 minutes, all liquid in the funnel was vacuum removed and
the nanowire network was separated from the PVDF membrane. After
being dried in an oven at 100 degrees Celsius for 15 minutes, the
nanowire network was submerged in 10 cc of 10 nm Au colloid
solution and soaked for 20 minutes to absorb the Au nanoparticles
on the surface of the nanowires. Finally, the nanowire network was
removed from the Au colloid solution, rinsed with isopropyl alcohol
(IPA), and dried at 100 degrees Celsius to obtain a nanowire
network coated with gold nanoparticles. FIG. 9 shows the SEM image
of the Au catalyst nanoparticles deposited on the network of
interconnected nanowires.
[0071] Although described in considerable detail above, it will be
appreciated that various modifications may be made to the
above-described invention, while still practicing the invention as
it is delineated in the appended claims. All publications and
patent documents cited herein are hereby incorporated herein by
reference in their entirety for all purposes to the same extent as
if each such document was individually incorporated herein.
* * * * *